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DEMI
Down East Microwave Inc.
Design Note
From: DEMI R&D Dept.
DN#: 014
Date: August 15, 2002
Re: Frequency Drift of DEM 432-28 Transverter
PREFACE: During extensive use such as EME , contesting, or long QSO’s, frequency drift of the
DEM 432-28 may occur. This drift may vary in severity depending on the version of transverter. The earlier
models will drift more that the newer ones. Voltage and temperature regulation of the oscillator circuit with
excessive internal air temperature are the main problems. Customer feed back and further engineering
have resulted the modifications discussed in this document.
SOLUTIONS: The earlier versions, pre-1998, can be upgraded to today’s standards by replacing the
voltage regulator, oscillator transistor, trimming capacitor and adding a PTC Thermistor to the oscillator
circuit. Some units may already contain a Thermistor attached to the crystal. The Thermistor cures most of
the drift but after extensive heating of the internal air in the transverter, it will drift up and down in frequency
as the temperature changes. For owners of these earlier versions, if you desire to attempt the
modifications, it is important to identify which version transverter you have. We suggest consulting DEMI
before you attempt a modification. We can supply the components and guidance which varies depending
on the version transverter you have. If the transverter is a factory built unit, we can modify it to today’s
standards for a nominal charge.
After the modifications have been completed or if you have a newer version of this transverter, extended
use of the 432-28 will still cause excessive heating of the air trapped inside of the enclosure. This increase
in air temperature will heat other related components in the oscillator circuit causing frequency drift. Then
when the air cools, the frequency will drift back. To control this problem we suggest an external cooling fan
mounted on the Heat Sink above the power amplifier section. If the frequency is stabilized at room
temperature with the cooling fan in operation, then the PTC Thermistor will have complete effect in
stabilizing the oscillator’s temperature. If the air temperature in the enclosure rises above the temperature
of the oscillator circuit, frequency drift will occur. If the Heat Sink is “Hot” to the touch when operating, it will
drift . Use more cooling air!
FURTHER EXPERIMENTATION: After all modifications are completed on the older versions of if you
have a newer model, you may experiment with vent holes in the enclosure. We have not thoroughly tested
this but see it as an improvement in future 432-28’s. If you attempt to use a cooling fan with vent holes, be
sure that the air does not directly move across the oscillator circuit. This would nullify the use of the PTC
Thermistor.
Down East Microwave, Inc
945 Rt 519 Frenchtown, NJ 08825 USA
Tel. 908-996-3584 (Voice)
908-996-3702 (Fax)
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